Dear Faizal,
It is nice to hear that you already started using it 
Regarding JRF question: The trunk joint reaction force in SelectedOutputs are now expressed in new reference nodes that are aligned with the vertebral body. This means that the forces are compression and shear forces. Therefore, the names of the outputs now include CompressionForce and ShearForce. The new reference
odes are created inside the joint node on the proximal segment. To compare with the previous version, it is just the coordinate system that is rotated to be aligned with the vertebra and the total reaction force will be the same.
In the previous version, the coordinate system The implementation from AMMR 2.4 can be enabled by using the compatibility switch:
#define BM_COMPATIBILITY_24_TRUNK_SELECTED_OUTPUTS_JRF ON
Regarding Spine Rhythm question: In the new thoracic model, there are 12 thoracic vertebrae. As default, we have spherical joints between the vertebrae, which leaves 36DOF to drive (3DOF for each joint). Basically, rhythms are the drivers for these needed DOFs. When the rhythm for thoracic spine is ON, the thoracic spine needs driver for 3DOF and the rest is already handled with the rhythm. With these 3DOFs you can drive the spine to flexion, extension, lateral bending or axial rotation. Note that there are rhythms for lumbar spine as well. In the default model, you can drive the whole spine by just 3DOF, which is the flexion, lateral, and axial angle of the whole spine
You can have RHYTHM_SOFT and also have C7 and T10 markers. It will try to find a spine posture in between the rhythms and the markers in the optimization process. You can remove the 3DOF drivers that drives the whole spine and use C7 marker driver. If you add T10 driver, then you are driving 3 more DOFs and because the rhythm is soft, it should find a solution.
I hope it helped.
Best regards,
Hamed